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Should I Change the Gear Ratio on my Bike?

Posted by Mikey Ducard on

It’s not just custom cycling wheels that will make a difference to your performance on the bike; it’s the gears too. Three decades has seen a huge progression in terms of the number of gears on bikes and we’re now at the point of 22 gear groupsets, thanks to the combination of two chainrings and 11 sprockets. What’s more, the variety in sizes of chainrings and sprockets has also evolved as well, therefore offering a wide range for riders to hit the sweet spot in terms of gearing options on their bike.

With such a generous number of gears on offer, riders might think that they should just stick instead of twisting, but they’d be wrong. Without trying different setups, riders may well not ever know if they’re riding on the ideal combination for their body. After all, there is a finite amount of power on offer and the efficiency of the system depends upon maintaining a consistent cadence (~80rpm), irrespective of the riding conditions.

Every road cyclist should know that the size of the chainrings and sprockets command the gearing of a bike, but there’s more than just the volume of teeth involved. Essentially, the key here is the ratio and the way the sprockets multiply the effort made with the chainring.

Doing the Numbers

You can work out the number of wheel revolutions produced by a bike’s gearing through knowing the ratio of the chainring to the rear sprocket, for example:

A 54T chainring is paired with a 13T cog; it has a ratio of 54:13, or 4.15. Making a simple complete rotation of the crank will cause the rear wheel to rotate 4.15 times.

Comparing Gear Ratios

Okay, so we’ve established that there are options galore at your disposal. Now we need to know how to compare them so you know what’s best to get you blowing up the sprints like Andre Greipel, or steaming up the climbs like Chris Froome.

Method One

Roll-out, or meters of development, is worked out by multiplying the gear ratio by the circumference of the wheel (measured in meters). This is basically the distance the bike will travel after one crank revolution. For example; using 53 x 12T gives you a return of 9.28 meters of roll-out for a road bike fitted with 25C tyres compared to 3.28 meters for 39 x 25T.

Method Two

For this method, you have to relate the gear ratio to wheel size by multiplying the gear ratio by the diameter of the wheel. Take a 27-inch road wheel, for example; using a high gear ratio such as 54 x 13T is comparable to riding a penny-farthing with a front wheel that is nearly 10 feet tall! In contrast, a low gear ratio like 38 x 26T is equal to a 42-inch wheel.

Of the two, roll-out is a little more informative, if only because it is more tangible than a theoretical wheel diameter. Nevertheless, either value can be used to easily calculate the expected speed for any given cadence:

Speed (miles/h) = Gear inches/63, 360 x Pi (3.14159) x cadence (rpm) x 60

Speed (km/h) = Roll-out/1000 x cadence (rpm) x 60

What’s more, it’s these kinds of considerations that could mean the difference between killing it in a fixed gear (e.g. track and Red Hook crit riders), in particular. All it takes is a negligible difference in gear ratios (0.1m/1 gear inch) to alter the ease that you can accelerate and the maximum speed you can hit. For road riders, these differences won’t be felt, and it’s the larger increments (0.5m/5 gear inches) that are more meaningful for you guys and girls.

For those riders that like to spin and can maintain a high cadence for long periods, compact chainrings are likely to suit them better than a standard combination. In contrast, riders that can push bigger gears at a lower cadence are more likely to prefer bigger chainrings.

Choose Your Gearing

The only way to decide how any of these effects on the gearing of a bike translate to the road is to put them to the test. This can be a costly exercise, especially when considering a change of chainrings, so the best time to explore the issue is when one or all of the parts of the transmission are due to be replaced.

If you’re thinking about heading to the French Alps, you can make some assumptions around the range of low ratios that are currently in use. Worst case scenario is wishing you had a lower gear, so it is well worth adding at least one extra low ratio so you can pedal without feeling like you’ve just ridden into setting concrete while breathing directly fire into your lungs.

For these kinds of rides, you won’t see much use in having high gear ratios, mostly because the descents will be unfamiliar, so you won’t be attacking them full on. Here’s where you can say bye to a couple of high ratios without affecting the utility of the bike. Even today, egos ride high and there is still far too much focus around the preservation of high gear ratios on road bikes. it’s nothing personal, but the majority of riders just don’t have the horsepower to push these gears unless you’re descending, but you can let gravity do its thing then anyway. So, be humble and get the right setup for your riding, not what looks closest to a pro ratio.

Any good local bike store will be able to help in your quest for the perfect ratio, but hopefully, this has given you some inspiration to try and find the ratio that best suits you…!

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